Modern vehicular powertrains incorporating an engine, such as an internal combustion engine, and a transmission, such as an automatically shiftable transmission, typically include a fluid coupling disposed therebetween. The fluid coupling operates to substantially decouple the engine from the transmission to allow the vehicle so equipped to come to rest without causing the engine to stall. Additionally, as in the case of a torque converter, the fluid coupling may operate to provide a measure of engine torque multiplication within the powertrain.
Further, the fluid coupling may serve to impede or damp the oscillations, caused by the firing pulses of the engine, from being transmitted to the transmission and other components within the powertrain. To increase the operating efficiency of the powertrain, some fluid couplings incorporate a lockup clutch, sometimes refereed to as a torque converter clutch (TCC). The lockup clutch is selectively engageable to substantially couple the engine to the transmission thereby reducing any slip that may be occurring within the fluid coupling. In order to make the engagement and disengagement, or release, of the lockup clutch imperceptible to the operator of the vehicle, careful attention is paid to provide optimal powertrain operating conditions during such operations.
The engine may provide an amount of engine braking when the vehicle is coasting or decelerating. This condition will cause the engine to “motor” or experience negative torque as a result of pumping losses. The operator of the vehicle may feel a shock or impulse should the lockup clutch be disengaged or released while the engine is experiencing a large amount of negative torque.